Ive got a 73 cougar convertible. It has a vibration that starts at about 60mph and stops at about 75mph. Its not a constant vibration, it oscillates. Ive replaced U joints and tail shaft bush. It only does it in 3rd gear (fmx trans) run it up in 2nd theres no vibration. It has to be transmission generated. Anyone have any ideas as to what may be causing it?
Could be the tailshaft bushing, transmission mount, or perhaps the drive shaft needs to be balanced. Look at all three and see which is in poor shape.
All are perfect. And if it were any of those then it would be there in 2nd gear which it isnt
You can do 75 in 2nd? Cool! Sound transmission related if it only does it in third